Humans have what is known as binocular vision owing to the fact that we have two eyes separated by a couple of inches. Each eye views the same scene from a slightly different perspective view each providing the brain with slightly different information. These two views are combined by the brain such that we perceive depth and see the world in three-dimensions (3D).
Electronically stored or transmitted visual images, such as pictures or video, is typically displayed on a two dimensional medium such as a television screen or other type of monitor or projected on to a screen. Both eyes view the same information. The brain is thus left to use other visual cues from the two-dimensional (2D) image, such as relative sizes of objects, shadow, perspective lines, or horizons, to name a few, to sense depth. However, the picture still looks flat and not like we see the real world.
Stereoscopy refers to any of various processes and devices for giving the illusion of depth from two-dimensional images. We say illusion because true 3D may be more like a hologram where you could walk around the image and change your perspective. However, when done correctly, stereoscopy can trick the brain into thinking objects are jumping out of the screen at you.
In its simplest form, two cameras, or one camera with two lenses, spaced a few inches apart, are used to capture two 2D images. Each 2D image, of course, is from a slightly different perspective such that when the left eye views one image and the right eye views the other, the brain combines the views and we see the combined image as three-dimensional (3D).
Big screen stereoscopic motion pictures or “3D movies”, as is the term more commonly used, are becoming quite popular again. In addition, 3D technologies are now available for home video with the so-called 3D TVs, video games, and streaming and recorded video content for computer monitor viewing.
There are several types of stereoscopic or “3D” technology available. Most require the viewer to wear special glasses or goggles. Some require active components in the glasses, others do not. Some require special monitors or drivers. Each has it pros and cons and, depending on the situation, may or may not make sense for a specific task.
Regardless of the technology used, the end goal is primarily to separate what the left and the right eye sees. Early technologies involved physical separation where a viewer looked into a binocular-like device, with a lens for each eye to physically separate the left and right views. This technique which may be the oldest, works quite well and a close variation of this technique is still used in modern virtual reality goggles or head-mounted displays. However, this is only good for one person or individual viewing and may be expensive or impractical for more than a couple viewers.
One of the first left/right (L/R) separation technologies good for the masses was spectral separation. The technical term is “color anaglyph” and involved each viewer wearing a pair of glasses with a red filter for one eye and a blue filter for the other. The left and right images were likewise blue or red encoded and displayed simultaneously. This technique was popular for producing 3D movies in the 1950s and even works to some degree with standard color televisions or monitors. While providing a novelty for its day, it left much to be desired aesthetically. The end result tended to be monochromatic, and had a lot of ghosting (i.e. the L/R separation was not clean). On the pro side, it was inexpensive to produce and the glasses were passive and very inexpensive.
Similar to spectral separation, the next most common technique is spatial separation and involves the viewers wearing polarized glasses, with each eye lens being polarized at 45 degrees, for example, to the other or circularly polarized in opposite directions. This is the technology used most often today in movie theaters. It works pretty well with the L/R separation being fairly complete, but usually requires two projectors or a special projector in a theatre setting or a few additional layers in a monitor which adds cost. Also, each eye only sees half resolution which may degrade the viewing experience. On the pro side, the polarized glasses are again passive and therefore relatively inexpensive.